1. Field of the Invention
The present invention relates to method for compensating displacements of a machine point due to temperature changes. The invention relates particularly to a method for compensating displacements due to temperature changes of a work spindle axis relative to a machine reference point. The measurement values for the intended positioning of the work spindle axis are determined by means of a position measuring device which includes at least one sensing device for sensing a measuring scale.
2. Description of the Prior Art
In newer types of machine tools, intended displacements in the directions of the individual axes are determined by means of position measuring devices or measurement systems and the measured values are digitally indicated by means of position indicators. The measured values determined by the position measuring devices can also be supplied to NC-controls for further processing.
Even though the determination of the positions on the individual axes in, for example, a universal tool and milling machine, can for structural reasons not be carried out directly at the location of the respective axis, the precision of the determination of the position is not negatively influenced as long as the operating temperature does not change non-uniformly.
A high rate of rotation of the work spindle during operation causes a rise in the operating temperature of the spindle bearing and of the vertical milling head in the vicinity of the axes relative to the spindle carriage of the milling head. However, the position measuring device for measuring the intended displacement of the spindle carriage and, thus, of the work spindle axis, is mounted on the spindle carriage.
Accordingly, due to the rise in temperature, the work spindle axis is displaced. This displacement is not detected by the position measuring device.
Various proposals have been made in the art for compensating these displacements due to temperature changes.
For example, German Pat. No. 2,450,322 discloses a solution in which a so-called expansion rod is utilized. Such expansion rods contact the measuring scales of the position measuring device. If such expansion rods are used, it is necessary to take into consideration the coefficients of expansion of the various materials used. In addition, the given machine geometry must be taken into consideration. While such systems in which expansion rod compensations are carried out are simple in structure, they have the disadvantage that they react relatively sluggishly to temperature changes.
Another solution is disclosed in German Offenlegungsschrift No. 3,320,529. A scale of the position measuring device is movably mounted on the spindle carriage and a push rod of a material having a small coefficient of expansion causes a displacement of the scale in accordance with the temperature change. Compared to the solution utilizing the expansion rod, the solution according to German Offenlegungsschrift No. 3,320,529 has the advantage that displacement of the work spindle axis due to temperature changes are taken into consideration with hardly any delay. However, this solution does not have the disadvantage that very exact guide means must be provided for the displacement of the scale.
It is, therefore, the primary object of the present invention to provide a simple means for compensating the displacement of a machine point due to temperature changes. In addition, position measuring devices are to be used for this purpose which are in general production or at least essentially in general production.
It is another object of the present invention to provide an apparatus for carrying out the above described method.